Coal as an energy source is in abundant supply.
The substitution of coal for natural gas and oil on a large scale would therefore appear to be an easy solution to our energy shortage.
Unfortunately, however, unlike oil and gas consumption, coal use is limited not by reserve or production capacity but rather by extraordinary industrial and regulatory difficulties in burning it in a convenient, efficient and environmentally acceptable manner.
A number of techniques are being explored to provide coal as a more useful energy source, for example gasification and high pressure hydrogenation.
Another technique is the technique whereby solid coal particles are dispersed in water to form coal-water mixtures. Such coal-water mixtures offer considerable advantages. They are more readily transported than the dry solid coal, are more easily stored and are less subjected to the risks of explosion by spontaneous ignition, the latter being a significant factor in coal handling.
In addition, providing coal in a fluid form can permit its burning in apparatus normally used for burning fuel oil. This can greatly facilitate the transition from fuel oil to coal as a primary energy source, another highly desirable result.
Various coal-water mixtures are described in the literature.
In U.S. Pat. No. 3,762,887, there is disclosed a dispersion of coal in an aqueous medium wherein the coal is ground to a defined range of particle sizes, a substantial portion of which being about 325 mesh Tyler Standard screen (44 micrometers) or even finer.
U.S. Pat. No. 4,217,109 discloses a technique for cleaning and dispersing coal in water utilizing dispersing agents which by selective adsorption impart different electrical charges to the carbon particles and the impurities. The dispersing agents taught are polyelectrolytes such as metal and ammonium salts of polycarboxylic acids and polyphosphates.
U.S. Pat. No. 4,358,293 discloses high coal concentration coal-water mixtures dispersed by non-ionic polyalkyleneoxide surfactants.
GB-A-2,099,451 teaches how to generate dense coal-water suspensions for pipeline transportation and direct firing of bituminous coal slurries in electric power plants.
Many other patents concern different, more effective, and cheaper dispersing agents.
The coal-water slurry fuels (CWF) are generated, as a rule, using high volatile bituminous coals. Generation of CWF from low rank coal, such as sub-bituminous and lignites, is not technically or economically feasible with existing technologies.
Low rank coals are defined as coals having a carbon content ranging from about 60% to 78% by weight (daf), and a relatively high oxygen content, ranging from about 16% to 25% by weight (daf). Other characteristics of low rank coals are a relatively high moisture content in the range of about 10% to 40%, a high dry ash content in the range of about 12% to 40%, volatile materials greater than about 38% (daf), fixed carbon of less than about 62% (daf), and about 1% to 10% of oxygen in form of process.
Most countries with LRC reserves have conducted research to develop a non-evaporative, high-pressure, continuous process to produce a dry product based on the principles of the Fleissner process.
Australian Patent 430,626 (1970) discloses a continuous hot-water coal drying process for brown coal in Australia.
Similar processes are described in U.S. Pat. Nos. 4,018,571, 4,052,169, 3,992,784, GB-A-1,471,949 and Canada 1,020,477. Commercialization of this technology, however, has lagged due to the difficulties in separating the dewatered product from the free water used to transport the coal through the process.
Since 1983, the University of North Dakota Energy Research Centre has been studying the hot-water coal drying process to produce CWF from LRC.
T. A. Potas, G. G. Baker and D. J. Maas, describe in the Journal of Coal Quality, April 1987, a pilot scale preparation of low rank coal-water fuels by hydrothermal treatment at a temperature between 270.degree. C. and 330.degree. C. This process suffers the drawback of a high pressure apparatus requirement. Moreover, product coals are inadequately upgraded, the equilibrium moisture and the oxygen content being too high for a good CWF. Finally, some CWFs are not very fluid and settled to form a hard pack in some hours.
Therefore, there remains in the art a need for a process which, operating under mild conditions, effectively upgrades low rank coal to yield fluid and stable coal-water fuels.